Gaza City (CNN) — As Israeli tanks and soldiers plunged deeper into Gaza, Prime Minister Benjamin Netanyahu warned Friday that the military is prepared to further expand its offensive against Hamas militants.

Israel’s military incursion into Gaza made for high casualties and a doubling of displaced Palestinians, to 40,000, prompting a United Nations relief agency to seek $60 million, authorities said.

Meanwhile, President Barack Obama said Friday he spoke with Netanyahu and reiterated U.S. support for Israel’s right to defend itself against Hamas rockets being fired from Gaza.

But Obama also said he hoped Israel’s ground assault on Gaza would continue to take an approach that “minimizes civilian casualties.”

The Israel Defense Forces sent a large force into Gaza on Thursday — infantry, tanks, artillery, combat engineers and intelligence units backed by aerial and naval support.

Their primary target: Tunnels used by Hamas fighters and others to slip into Israel and to smuggle in weaponry and supplies.

Netanyahu did not explain what would spark a wider offensive, or what it would entail. But he said Israel had no choice but to take the fight to Gaza to protect its own people.

“We chose to commence this operation after we had exhausted the other possibilities,” he said, “and with the understanding that without action, the price that we would pay would be much greater.”

Hamas spokesman Sami Abu Zhuri condemned the Prime Minister.

“Netanyahu is killing our children, and he will pay the price. The ground invasion is not scaring us. We pledge to drown the occupation army in Gaza’s mud,” Abu Zhuri said.

The casualties

The violence between Israeli forces and Hamas militants has made for high casualties.

The Palestinian Health Ministry said that at least 271 Gaza residents have died and more than 2,000 have been injured in the territory since Israel began its military campaign against Hamas last week. At least 24 of the deaths occurred since the ground offensive started late Thursday, the ministry said.

Most of the casualties have been civilians, according to the United Nations.

The IDF said early Friday that one Israeli soldier was killed overnight in northern Gaza — the second Israeli fatality of the conflict. The IDF said its troops had killed at least 17 fighters and captured 13, in addition to uncovering 13 tunnels.

Within Gaza, the number of displaced people grew Friday to 40,000 from 22,000, and the United Nations Relief and Works Agency was operating 34 shelters, a spokesman said. The agency launched an appeal for $60 million to assist the displaced for a month and then provide provisions for the next six months, the spokesman said.

The fighting

Early Friday, artillery fire pounded Beit Hanoun in Gaza. The area was shrouded in smoke.

A CNN crew near Sderot, Israel, spotted a substantial increase in armor and tanks on Friday.

Hamas’ military wing, the Al Qassam Brigades, planted a bomb that exploded in the path of an Israeli tank east of Khan Younis in southern Gaza, Al-Aqsa TV reported.

The energy authority warned late Friday it might cut electricity to most areas later in the night, Al-Aqsa TV reported.

Roads leading into Gaza were crowded with military traffic and buses carrying soldiers.

Palestinian security sources said Israeli tanks had reached Abu Holeh by Friday, located roughly in the center of Gaza, and that Israeli troops were clashing with Hamas and Islamic Jihad fighters along the Kissufim Road.

If Israeli forces go from there to the Mediterranean Sea, they could split Gaza as they did during their 2009 ground operation in the territory.

The incursion Thursday followed 10 days of Israeli airstrikes. the collapse of a cease-fire brokered by Egypt but never accepted by Hamas, and a five-hour respite Thursday to allow for humanitarian assistance to reach Gaza.

The protests

The Israeli operation set off some protests around the world, including in Turkey — where violent demonstrations outside the Israeli Embassy prompted Israel’s Foreign Ministry to send diplomats’ families home and reduce staffing to a minimum.

In Jerusalem, police arrested 12 people Friday after what spokesman Micky Rosenfeld described on Twitter as “masked Arab rioters” threw stones at police officers on the Temple Mount, a disputed holy site. No injuries were reported, Rosenfeld said.

Jordan called for an urgent meeting of the U.N. Security Council, expected Friday.

And Hamas condemned the incursion. “The beginning of the Israeli ground invasion of Gaza is a dangerous step with unknown consequences. Israel will pay a heavy price for it,” spokesman Fawzi Barhoum told CNN.

Barhoum said Hamas military forces are “far stronger” than during previous conflicts with Israel in 2009 and 2012. Militants are prepared to capture Israel soldiers and use them to trade for some of the 5,000 prisoners in Israeli jails, Barhoum said.

“Where are we supposed to go?”

Al-Aqsa TV reported Friday that Israel had sent text messages to many Palestinians telling them of safe corridors to reach central Gaza.

Before the incursion, the IDF dropped leaflets in 14 areas of Gaza, urging residents to temporarily leave their homes.

“The IDF is a moral army, and it does not aspire to harm even one single innocent person,” Netanyahu said Friday. “Not a single one. We are only operating against terrorist targets.”

But many residents of Gaza have said they have nowhere to go in the small, impoverished strip of land. Border crossings with Israel and Egypt are closed.

“Since the Israelis started this 11 days ago, they have been telling us to leave. Where our we supposed to go — to the Gaza Sheraton? Or take a hike in the forest?” said Al Madhoun, the resident of northern Gaza.

Ramez al Madhoun, a resident of Beit Lahiya in northern Gaza, told CNN by phone that people were streaming past his house Thursday night to flee the Israeli offensive.

As blasts from airstrikes and artillery barrages went off in the background, Al Madhoun said that he and his family of about 20 people were staying put.

“My father is 78 years old — where am I supposed to go?” he asked. “We are a sitting duck.”

Officials call for restraint and precision

Israeli officials said they were focused on destroying tunnels used by Hamas and other groups in Gaza to smuggle supplies in and out of the territory.

U.S. Secretary of State John Kerry spoke with Netanyahu after the offensive began, expressing the view that it “should be a precise operation to target tunnels,” the State Department said in a statement.

Kerry “emphasized the need to avoid further escalation” and reiterated “the importance of doing everything possible to prevent civilian casualties,” the statement said.

Chris Gunness, spokesman for the United Nations Relief and Works Agency for Palestine Refugees, said the group was asking for restraint “so that civilians who have suffered enough in this appalling conflict do not suffer further.”

The U.N. agency said it is sheltering about 22,000 people in Gaza City and northern Gaza.

Negotiators were focusing on stopping bloodshed above all else, Palestinian official Nabil Shaath said at the time. He said they would later discuss Hamas demands, including opening Gaza border crossings and freeing some Palestinian prisoners.

Hamas leaders had rejected an earlier Egyptian cease-fire proposal, saying they had not been consulted on the deal and complaining that it did not address their broader demands.